1. Field of the Invention
The present invention relates generally to heat transfer material and more particularly to one that is thermally conductive, but electrically non-conductive. Articles made with the material include a dielectric jacket for a heating cable, and a thermally conductive, but electrically non-conductive, jacket for a steam/fluid tracer tube as well as thermally conductive strips for bridges between steam/fluid/electrically heated tubes and process piping, vessels, and equipment.
2. Description of the Related Art
The use of thermally conductive materials in heat tracing applications is known in the art. As early as 1954, filled thermally conductive materials were being commercially used in industrial heat tracing applications. Early heat transfer materials for heat tracing used carbon based fillers, such as graphite, loaded into a receiving base material such as sodium silicate, epoxy, etc. These materials were applied in paste form to the exterior of a tube through which steam was passed. The passage of steam through the tube caused the water in the sodium silicate to evaporate. This resulted in the heat transfer material hardening and thus permanently and physically bonding the steam tube to the process pipe to which it was mounted. This physical bonding enhanced the heat transfer between the steam tube and the process pipe and thus resulted in much higher maintenance temperatures on the process pipe than would be experienced by traditional steam tracing methods using no heat transfer material for a given steam/fluid temperature.
In 1974, Bilbro et al. obtained U.S. Pat. No. 3,834,458 for a new heat transfer material which achieved similar results as prior heat transfer materials but allowed for a partially cured conductive material to be snapped in place over the tube and then covered with a containing channel. The advantage here was the heat transfer material became molten and flowed to fill air gaps after steam was passed through the tube. The installation of the heat transfer material was cleaner and faster. The conductivities of the cured heat transfer material of the ""458 patent were only slightly less than the previous paste-like heat transfer materials. The heat transfer material disclosed in the ""458 patent was also extensively used with electric heat tracers by extruding the heat transfer material onto the electric cable at the factory and then shipping the electric heat tracer to the field on a reel. In the field, the electric cable with the extruded heat transfer cement was installed on a pipe and again covered with a channel.
In recent years, certain applications have been identified where it is not possible to keep the extruded heat transfer cement material, as disclosed in the ""458 patent, always beneath a channel. One specific application is the rail heating application. Specifically, when used with rail heating, the electric cable heater has to leave the rail at expansion joints and then after a one or two foot loop return to heat the rail. The prior art heating cable included an extruded thermally conductive and electrically conductive heat transfer material. The heat transfer material contained carbon black, which provides the required thermal conductivity, however, it is also highly electrically conductive.
Since the prior art heat transfer material was electrically conductive, it posed a hazard for electrical shock. Thus, in the past, a thin silicone rubber jacket has been placed around the extruded heat transfer cement material to retain its shape at the excursion points of the heater cable from the rail. Since the rails in many cases were electrically alive (480 to 800 volts DC or AC potential), the silicone jacket material provided electrical insulationxe2x80x94should anyone brush against these loop arounds. Materials other than silicone have also been used for this purpose, one of which is described in U.S. Pat. No. 4,391,425, issued to Keep.
Many other applications also require dielectric jackets, so electrical conductivity of prior art heat transfer materials is often a problem. Due to the composition of the prior art heat transfer material used, the heat transfer material would cure and harden when placed into service. Consequently, prior art heating cable was typically not reusable after it was removed from a heated surface because it became hard and brittle during service. In the rail heating application, when rail replacement was necessary, it also became necessary to replace the heating cable.
Similarly. heat transfer material that has been extruded onto a steam/fluid tracer tube and installed under a channel typically cannot be subsequently removed and reinstalled without damaging the heat transfer material. Most prior art heat transfer materials for steam/fluid tracing bond or adhere to some extent to the heated surface when in service, which again prevents reuse. Where heat transfer material has been used between two tubes, which have high expansion forces. the expansion forces have caused the material to crack.
A thermally conductive, but electrically non-conductive, heat transfer material is provided according to the present invention. For example, a jacket or insulation layer is provided for heating cables for rail heating applications, electric heating and power cables, jacketed steam/fluid tracer tubes, and removable/reusable thermal bridge strips for heat tracing tubes. The thermally conductive, but electrically non-conductive, articles so made are mechanically sturdy, but flexible. Cable, tubes, bridge strips and similar articles can be shipped on a reel to the final destination. A thermally conductive material for heat transfer devices is provided that retains flexibility after use, which has dielectric properties. Articles made with the present heat transfer material do not pose an electrical shock; do not become hard and brittle after use; and do not become bonded to the surface. Yet, the material meets thermal conductivity requirements.
The thermally conductive, electrically non-conductive composition comprises a polymeric material, such as silicone rubber, and a nitride and/or oxide compound as a filler material. Suitable nitride and/or oxide compounds include, but are not limited to, aluminum nitride, boron nitride, silicon nitride, aluminum oxide and beryllium oxide. Compounds that are chemically or physically similar to the specified nitride and oxide compounds may be suitable as well. Preferably, additional plasticizer additives are included to increase the flexibility of the jacket material. The jacket material of the present invention has a thermal conductivity that approaches the thermal conductivities of prior art heat transfer materials, is not electrically conductive, and remains flexible at temperature exposures up to and exceeding 450xc2x0 F. and does not harden or adhere to the substrate.
A heating cable has a thermally conductive, electrically non-conductive jacket. Such a cable can be installed on a third rail that is usually electrically alive with 480 volts to 800 volts DC or AC potential. The heating cable with a jacket according to the present invention can be installed on a live third rail without a danger of electrical shock to the installer. The thermally conductive, electrically non-conductive jacket will not form a galvanic corrosion (cell) on the carbon steel third rail. The jacket can be extruded onto the cable during manufacture.
A heating cable according to the present invention, with a thermally conductive, electrically non-conductive jacket, can be used in electric heat tracing applications, where reduced element and conductor operating temperatures are advantageous. A composition of material according to the present invention is also useful as a thermally conductive jacket for steam/fluid tube tracers or panels or thermal bridge strips between tracers and the heated surface which allows high heat transfer rates but allows the tracer to be removed and reapplied without sustaining damage to the heat transfer material.
It is desirable to have an improved thermally conductive, electrically non-conductive jacket for heating cable for rail heating applications. It is further desirable to have a thermally conductive jacket for a heat transfer element that retains flexibility after use. Many types of heating and power cable products require dielectric jackets. It would be advantageous for these heating cables to be jacketed with a highly thermally conductive material in order to reduce the inner conductor/element operating temperature. As these jackets are dielectric jackets, they should remain essentially electrically nonconductive. A heat transfer material according to the present invention or jacket, sheath, strip, insulator or covering made of it addresses these desires.